1. Field of the Invention
The present invention relates to the processing of kaolin clays, and more particularly pertains to the production of high grade, refined kaolin clay pigments useful as paper coating pigments from crude kaolin clay ores having high inherent aqueous slurry viscosities.
2. Description of the Prior Art
As known in the art, variations in the properties of crude kaolin clay ores are common and are the result of such factors as the mineral composition of the ore, its particle size or shape, the particle size distribution, and the like. Within certain defined limits it is possible to alter the properties of kaolin clays by certain processing techniques, such as fractionation or classification, delamination, treatment with chemical additives, such as leaching agents, flocculants, dispersants, etc. Thus, it has become common practice in recent years in the production of refined high grade kaolin clays, such as that required in coating paper, to deflocculate the crude ore in an aqueous suspension, to classify and fractionate the crude into fine and coarse fractions and thereafter to delaminate, leach, and/or otherwise treat the crude fractions with various chemicals and/or additives to produce a finished clay product. More detailed discussions of such known processes are disclosed in U.S. Pat. Nos. 3,301,691; 3,371,988; 3,320,027; and 3,442,677, to name a few.
Notwithstanding the fact that there are many known and commercially advantageous processes for refining kaolin clays, there remains a significant problem in this area. This problem lies in the fact that the clay-water viscosity is an important factor of whether or not a clay may or may not be used in a given application or end use. One of the most important uses of kaolin clay is as a pigment in coating paper to produce a desired finish or gloss. In conventional paper coating techniques, the clay pigment in the form of an aqueous slip or slurry is normally employed in a coating formulation which is applied to the paper. However, in the paper industry, very high machine speeds are required to apply this coating. Also, in order to obtain adequate drying rates concentrated slurries of the clay pigment are used. Therefore, the viscosity of the clay slip is particularly significant, and paper coating grade kaolin clays are sold under certain rigid viscosity specifications.
In this regard a large amount of Georgia kaolin clay reserves suitable from the standpoint of color and purity for refining into paper coating pigments is at a viscosity too high for such commercial use. More particularly, such crude kaolin clay ores have such high native or inherent clay/water viscosities that they are incapable of being refined into acceptable paper coating clay pigments in accordance with conventional clay pigment refining procedures. As a result it has heretofore been necessary, in mining kaolin clays for producing paper coating pigments, to reject those portions of the clay ore whose native viscosity is at an undesirably high level. While prior art workers have expended much time and effort in attempting to solve this problem none have proved to be totally successful so far as is known, and at the present, high viscosity kaolin crudes are usually rejected for producing paper coating clay pigments.